The present invention relates to plate-like ferrite particles with magnetoplumbite structure and a magnetic card using the plate-like ferrite particles with magnetoplumbite structure. More particularly, the present invention relates to plate-like ferrite particles with magnetoplumbite structure having an appropriate particle size, a low coercive force, a large saturation magnetization, a small switching field distribution (S.F.D.) and an excellent temperature stability, and a magnetic card containing the plate-like ferrite particles with magnetoplumbite structure.
In recent years, magnetic cards have been widely used in various application fields such as, typically, magnetic stripe-type credit cards, railroad tickets, season tickets, highway passes, telephone cards and prepaid railway cards.
These magnetic cards have been generally produced by a method of directly applying a magnetic coating material obtained by kneading magnetic particles and a binder together, to a card substrate, a method of applying the magnetic coating material to a non-magnetic substrate such as a base film and drying the applied magnetic layer to obtain a magnetic tape, and then attaching the obtained magnetic tape on a card substrate through adhesive, or the like.
Recently, there has been an increasing demand for high-performance magnetic cards. In order to produce such high-performance magnetic cards, it is important to strictly select magnetic particles used therein. That is, the magnetic particles having not only an appropriate particle size, a low coercive force and a large saturation magnetization, but also a switching field distribution as low as possible and an excellent temperature stability, have been strongly required.
The requirements concerning these properties of the magnetic particles are explained below.
The surface properties of a coat of the magnetic cards have a close relationship with the particle size of the magnetic particles. When the particle size is more than 1.50 .mu.m, the surface properties of the coat are adversely affected. Therefore, magnetic particles having a particle size of usually about 0.30 to about 1.50 .mu.m, have been conventionally used.
In addition, the coercive force of a magnetic layer of the magnetic card has a close relationship with that of the magnetic particles. As magnetic particles for the magnetic cards, there have been widely used plate-like ferrite particles with magnetoplumbite structure, because these particles can exhibit a coercive force as high as 2,000 to 3,000 Oe and can be readily magnetically oriented upon the preparation of a magnetic coating material.
However, from the standpoint of preventing a forgery or an alteration of magnetic cards, there have been recently used magnetic cards obtained by forming upper and lower magnetic layers whose coercive forces are different from each other, on a non-magnetic substrate.
That is, in Japanese Patent Application Laid-open (KOKAI) No. 4-291021(1992), there is described a magnetic card in which a low-coercive force layer having a coercive force of about not more than 1,400 Oe and a high-coercive force layer having a coercive force of not less than 2,900 Oe, are formed on a substrate.
In order to form such a low-coercive force layer having a coercive force of about 1,400 Oe or lower, it is necessary to use magnetic particles having a coercive force of about 1,400 Oe or lower. Further, in order to produce plate-like ferrite particles with magnetoplumbite structure having a coercive force of about 1,400 Oe or lower, metal elements such as Ti, Co, Zn and Sn have been conventionally incorporated therein to reduce the coercive force thereof.
In addition, when the plate-like ferrite particles with magnetoplumbite structure are used for the magnetic cards, it is necessary that the saturation magnetization thereof is as large as possible. This fact is described, for example, in Japanese Patent Application Laid-open (KOKAI) No. 56-149328(1981) as " . . . it is required that the magneto-plumbite ferrite particles used as a magnetic material for magnetic recording media have as large a saturation magnetization as possible . . . ".
Also, in order to improve a reproduction output from the magnetic cards, it is required that the switching field distribution (S.F.D.) is as small as possible.
That is, as described in Japanese Patent Application Laid-open (KOKAI) No. 63-26821(1988), when the relationship between the S.F.D. value of the magnetic recording medium and the reproduction output therefrom is graphed, it is recognized that the reproduction output is linearly increased as the S.F.D. value is decreased. This indicates that the use of magnetic particles having a small S.F.D. value results in increase in the reproduction output.
Further, there has been an endless demand for improvement of the properties of the ferrite particles with magnetoplumbite structure when applied to magnetic cards. For instance, it has been strongly required that the plate-like ferrite particles with magnetoplumbite structure have an excellent temperature stability.
However, as is apparent, for example, from FIG. 4 on page 1123 of "IEEE TRANSACTIONS ON MAGNETICS", HAG-18, NO. 6, it has been conventionally pointed out that the plate-like ferrite particles with magnetoplumbite structure have such a problem that the coercive force thereof is increased with the elevating in temperature, thereby causing deterioration in magnetic stability (especially, a stability of coercive force) with respect to temperature (hereinafter referred to merely as "temperature stability").
The magnetic recording on the magnetic card is performed by magnetizing the magnetic particles contained in the magnetic card in a magnetic field produced by a magnetic head thereon. In the case where magnetic particles having a deteriorated temperature stability are used in the magnetic card, even though the magnetic field produced by the magnetic head is so adjusted as to impart an optimum coercive force to the magnetic particles at room temperature, optimum recording on the magnetic card cannot be necessarily achieved when the coercive force of the magnetic particles is fluctuated due to the elevating in temperature caused by the change in environmental conditions.
Hitherto, as the ferrite particles with magnetoplumbite structure which show a small coercive force, there have been known plate-like ferrite particles with magnetoplumbite structure which are obtained by substituting a part of Fe therein with metal elements other than Fe, such as Co--Ti, Zn--Ti, Co--Sn or Co--Ti--Sn.
For example, there have been known plate-like ferrite particles with magnetoplumbite structure for magnetic cards, containing Bi, Zn--Ti and the like (Japanese Patent Publication (KOKOKU) No. 7-106910(1995)), plate-like ferrite particles with magnetoplumbite structure, containing Bi and Co--Sn or Co--Ti--Sn (Japanese Patent Application Laid-open (KOKAI) No. 6-231930(1994)), or the like.
Presently, as the particles for the magnetic card, there have been strongly demanded plate-like ferrite particles with magnetoplumbite structure having not only an appropriate particle size, a small coercive force and a large saturation magnetization, but also a small value of switching field distribution (S.F.D.) and an excellent temperature stability. However, plate-like ferrite particles with magnetoplumbite structure which satisfy all of these requirements have not yet been obtained.
More specifically, with respect to the above-mentioned conventional ferrite particles with magnetoplumbite structure in which Fe(III) is partially substituted with Co--Ti, the coercive force thereof can be effectively reduced even when the amount of Fe(III) substituted is small. That is, the plate-like ferrite particles with magnetoplumbite structure can exhibit the desired low coercive force while maintaining a large saturation magnetization thereof.
However, in the case where a part of Fe(III) contained in the particles is substituted with Co--Ti, there arises a problem that the magnetic properties thereof are fluctuated with the elevating in temperature. Especially, the temperature stability as to the coercive force of the particles is deteriorated to such an extent that the change in coercive force with temperature is not less than +4 Oe/.degree. C. Therefore, the plate-like ferrite particles with magnetoplumbite structure are unsuitable as magnetic particles for magnetic cards.
In addition, with respect to the above-mentioned conventional plate-like ferrite particles with magnetoplumbite structure which contain Bi and in which a part of Fe(III) therein is substituted with Zn--Ti, it has been difficult to effectively reduce the coercive force thereof. Therefore, in order to adjust the coercive force to an appropriate low value as required, it is necessary to substitute a large part of Fe(III) with Zn--Ti. This disadvantageously increases the content of the elements which are not concerned with magnetization of the particles, thereby failing to provide plate-like ferrite particles with magnetoplumbite structure having a large saturation magnetization.
Further, the temperature stability of these particles are still unsatisfactory though it can be slightly improved as compared to the particles in which a part of Fe(III) thereof is substituted with Co--Ti.
Furthermore, in the case where the plate-like ferrite particles with magnetoplumbite structure contains Bi and a part of Fe(III) therein is substituted with Co--Sn or Co--Ti--Sn, the coercive force of the particles can be effectively reduced even though the amount of Fe(III) substituted is small. As a result, there can be obtained plate-like ferrite particles with magnetoplumbite structure having a low coercive force as required while maintaining a large saturation magnetization. However, as described in Comparative Examples hereinafter, there arises another problem that the switching field distribution (S.F.D.) thereof is deteriorated. In addition, the temperature stability thereof is still unsatisfactory though it can be slightly improved as compared to those in which a part of Fe(III) therein is substituted with Zn--Ti.
In consequence, it has been presently demanded to provide plate-like ferrite particles with magnetoplumbite structure for magnetic cards, which have not only an appropriate particle size, a low coercive force and a large saturation magnetization, but also a small switching field distribution (S.F.D.) and an excellent temperature stability.
As a result of the present inventors' earnest studies, it has been found that by mixing a barium compound and/or a strontium compound, iron oxide, a bismuth compound, a zinc compound and at least one compound selected from the group consisting of a Nb compound, a Ta compound and a Sn compound, with each other at a specific weight ratio, and heat-treating the resultant mixture at 900 to 1,200.degree. C., the obtained plate-like ferrite particles with magnetoplumbite structure of a specific composition, have an appropriate particle size as desired, a small coercive force, a large saturation magnetization, a small value of switching field distribution (S.F.D.) and an excellent temperature stability. The present invention has been attained based on the finding.